1. Field of Invention
The present invention relates to an information device, and more particularly to enabling more rapid resumption of printing operations of an information device having an off mode in which limited power is consumed, when such device is turned on.
2. Description of Related Art
Some printers and other so-called small information devices operate using DC power supplied from an externally connected power supply device. In order to reduce power consumption, demand has grown for information devices that cut off the supply of DC power to internal circuitry including the CPU, or more particularly to components with relatively high power consumption, when the power switch is on but the information device is not used for an extended time and the main functions are not needed.
The European Union's Framework Directive on Eco-Design of Energy-Using Products requires that power consumption be limited to 0.5 W or less when in the off mode, that is, an operating state in which a device is connected to a commercial power source but no device functions are used.
Information devices with an off mode may have a switch circuit that interrupts the supply of DC power to internal circuits, and a latch circuit that holds the switch circuit open, as a power supply cutoff switch for cutting off the supply of DC power to internal circuits. Such information devices may also have a smoothing capacitor for stabilizing the supply voltage, and a discharge circuit including a discharge resistance for discharging any residual charge in the smoothing capacitor as necessary. Note that a capacitor with relatively high capacitance is used as the smoothing capacitor.
When the supply of DC power to the internal circuitry is cut off in such an information device, a start latch signal is input from the CPU to the latch circuit and the latch circuit latches the start latch signal, for example. While the start latch signal is latched, the latch circuit outputs a switch control signal to the switch circuit to hold the switch circuit open. While this switch control signal is input, that is, while the latch circuit latches the start latch signal, the switch circuit remains open and interrupts the supply of DC power to the internal circuitry. In order to resume supply of DC power to the internal circuits from this mode, the power switch of the information device must first be turned off and then turned on again. This is described below.
More specifically, if the power switch of the information device is turned off when the switch circuit is open and the supply of DC power to internal circuits is interrupted, the residual charge in the smoothing capacitor is discharged by the operation of the discharge circuit. When the voltage between the end terminals of the smoothing capacitor drops below the operating voltage of the latch circuit as a result of the residual charge being discharged, the latch circuit unlatches the start latch signal. When the start latch signal is unlatched, the switch circuit closes, and when the power switch turns on again DC power is supplied to internal circuits including the CPU.
See, for example, Japanese Unexamined Patent Appl. Pub. JP-A-2006-166561 and Japanese Unexamined Patent Appl. Pub. JP-A-2005-312162.
This means that the user of the information device may turn the power switch off and then immediately turn the power switch on again in order to resume supplying DC power to the internal circuits as described above. When this happens, however, the residual charge in the smoothing capacitor may not be sufficiently discharged and the latch circuit may not unlatch. As a result, DC power cannot be supplied to the internal circuits and the internal circuits cannot operate because the switch circuit remains open even though the power switch was turned on. To eliminate such problems, the time required to unlatch (the unlatch time) may conceivably be shortened by using a resistor with the lowest possible resistance and the greatest possible allowable loss as the discharge resistor disposed in the discharge circuit.
However, because the external dimensions of resistors with such characteristics are relatively large, using such a resistor as a discharge resistor leads to an increase in the size of the information device. Resistors with such characteristics are therefore not desirable for use in such information devices.